- extend `FD_SET()` hack to all platforms (was only Cygwin).
  Warnings may also happen in other envs, e.g. OmniOS.
  Ref: https://github.com/libssh2/libssh2/actions/runs/8854199687/job/24316762831#step:3:2021

- tidy-up `CURLcode` vs `int` use.

- cast an unsigned to `long` before passing to `curl_easy_setopt()`.

Cherry-picked from #13489
Follow-up to 3829759 #12489
Closes #13501

Warn by default in all builds, but without triggering error.

Once all new warnings revealed by CI are cleared, we can drop the
`-Wno-error=sign-conversion` options in cmake/autotools.

Follow-up to3829759bd042c03225ae862062560f568ba1a231 curl#12489
Closes #xxxxx

Before this patch, cmake/autotools builds made an exception for this
warning to not cause an error.

The codebase is warning-free now, so this patch deletes this exception.

Follow-up [...]
Follow-up to3829759bd042c03225ae862062560f568ba1a231 curl#12489
Closes #xxxxx

Change mingw-w64 printf format checks in public curl headers to use
`__MINGW_PRINTF_FORMAT` instead of `gnu_printf`. This syncs the format
checker with format string macros published via `curl/system.h`. (Also
disable format checks for mingw-w64 older than 3.0.0 (2013-09-20) and
classic-mingw, which do not support this macro.)

This fixes bogus format checker `-Wformat` warnings in 3rd party code
using curl format strings with the curl printf functions, when using
mingw-w64 7.0.0 (2019-11-10) and older (with GCC, MSVCRT).

It also allows to delete two workaounds for this within curl itself:
- setting `-D__USE_MINGW_ANSI_STDIO=1` for mingw-w64 via cmake and
  configure for `docs/examples` and `tests/http/clients`.
  Ref: c730c85 #14640

The format check macro is incompatible (depending on mingw-w64 version
and configuration) with the C99 `%z` (`size_t`) format string used
internally by curl.

To work around this problem, override the format check style in curl
public headers to use `gnu_printf`. This is compatible with `%z` in all
mingw-w64 versions and allows keeping the C99 format strings internally.

Also:
- lib/ws.c: add missing space to an error message.
- docs/examples/ftpgetinfo.c: fix to use standard printf.

Ref: #14643 (take 1)
Follow-up to 3829759 #12489

Closes #14703

We decided last year not to pursue avoiding this warning, because it
adds noise and friction, while in most cases not revealing actual code
issues. We fixed the interesting portion of them throughout mid-2024.

Conclude this effort by deleting related FIXMEs and temporary comments.

Follow-up to 3829759 #12489
Closes #15939